Some 50th and Christmas spoilers and casting information (some of this is probably known already)
The Day of the Doctor:
Peter de Jersey is playing Captain Androgar
Christmas special (currently untitled):
Sheila Reid (from Benidorm) is playing Clara's grandmother - she also played Etta in Vengeance on Varos
Orla Brady is playing Tasha Lem, a Dalek-human hybrid, like the ones in Asylum of the Daleks (eyestalks on their heads etc)
Jack Hollington is playing Barnable
Rob Jarvis is playing Abramal
Tessa Peake-Jones (Raquel from Only Fools and Horses) is playing Marta
The episode is basically the conclusion of series 6's story arc - "On the fields of Trenzalore, at the fall of the Eleventh, when no living creature may speak falsely or fail to give answer, a question will be asked. A question that must never, ever be answered."
The town called Christmas is on Trenzalore, and it has a truth field
The Daleks are the main baddie of the episode, somehow getting all their knowledge back of The Doctor
Lots of recurring monsters are back; Cybermen (including one made out of wood), Sontarans, Weeping Angels and Cherubs, Silence.
We know that The Question is "Doctor Who?". We also find out why The Silence want to prevent the question being answered, and why all of The Doctor's enemies formed an Alliance in the first place - it was always going to be something big to make them all come together like that. Whether we find out the answer to the Question, I don't know, but I'd say not. But what we do find out is what happens when it is answered:
The Time Lock is broken, and the Time Lords come back, and the Time War is unleashed again. That's what Matt's era has all been about. That's what brought the universe's most feared together - they don't want to return to that.
Plus, at Christmas, the Doctor has a "pet" Cyberhead which he calls Handles. Company over the hundreds of years, I guess!